Calcium mobilisation following shell damage in the Pacific oyster, Crassostrea gigas

Deciphering the molecular toolkit: regulatory elements governing shell biomineralization in marine molluscs

The intricate process of shell biomineralization in marine molluscs is governed by a complex interplay of regulatory elements, encompassing secretomes, transporters, and noncoding RNA. This review delves into recent advancements in understanding these regulatory mechanisms, emphasizing their significance in elucidating the functions and evolutionary dynamics of the molluscan shell biomineralization process. Central to this intricate orchestration are secretomes with diverse functional domains, selectively exported to the extrapallial space, which directly regulate crystal growth and morphology. Transporters are crucial for substrate transportation in the calcification and maintenance of cellular homeostasis. Beyond proteins and transporters, noncoding RNA molecules are integral components influencing shell biomineralization. This review underscores the nonnegligible roles played by these genetic elements at the molecular level. To comprehend the complexity of biomineralization in mollusc, we explore the origin and evolutionary history of regulatory elements, primarily secretomes. While some elements have recently evolved, others are ancient genes that have been co-opted into the biomineralization toolkit. These elements undergo structural and functional evolution through rapidly evolving repetitive low-complexity domains and domain gain/loss/rearrangements, ultimately shaping a distinctive set of secretomes characterized by both conserved features and evolutionary innovations. This comprehensive review enhances our understanding of molluscan biomineralization at the molecular and genetic levels.

Unravelling the ecotoxicological impacts of gadolinium (Gd) on Mytilus galloprovincialis embryos and sperm in seawater: A preliminary study

As the demand for rare earth elements (REEs) continues to surge in diverse industrial and medical domains, the ecological consequences of their ubiquitous presence have garnered heightened attention. Among the REEs, gadolinium (Gd), commonly used in medical imaging contrast agents, has emerged as a pivotal concern due to its inadvertent introduction into marine ecosystems via wastewater release. This study delves into the complex ecotoxicological implications of Gd contamination, focusing on its impact on the embryonic development and sperm functionality of Mytilus galloprovincialis. The findings from this study underscore the potential hazards posed by this rare element, offering a critical perspective on the ecological risks associated with Gd. Notably, this exploratory work reveals that Gd exerts a significant embryotoxic effect at elevated concentrations, with an observed half maximal effective concentration (EC50) value of 0.026 mg/L. Additionally, Gd exposure leads to a considerable reduction in sperm motility and alters sperm morfo-kinetic parameters, especially at a concentration of 5.6 mg/L. The results highlight a dose-dependent relationship between Gd exposure and the prevalence of specific malformation types in Mytilus embryos, further providing crucial insights into the potential risks imposed by this rare earth element.

Vibrio cyclitrophicus population-specific biofilm formation and epibiotic growth on marine copepods

Vibrio spp. form a part of the microbiome of copepods-an abundant component of marine mesozooplankton. The biological mechanisms of the Vibrio-copepod association are largely unknown. In this study we compared biofilm formation of V. cyclitrophicus isolated from copepods (L-strains related to other particle-associated strains) and closely related strains originating from seawater (S-strains), and visualized and quantified their attachment and growth on copepods. The S- and L-strains formed similar biofilms in the presence of complete sea salts, suggesting previously unknown biofilm mechanisms in the S-strains. No biofilms formed if sodium chloride was present as the only salt but added calcium significantly enhanced biofilms in the L-strains. GFP-L-strain cells attached to live copepods at higher numbers than the S-strains, suggesting distinct mechanisms, potentially including calcium, support their colonization of copepods. The cells grew on live copepods after attachment, demonstrating that copepods sustain epibiotic V. cyclitrophicus growth in situ. The results demonstrate that in spite of their 99.1% average nucleotide identity, these V. cyclitrophicus strains have a differential capacity to colonize marine copepods. The introduced V. cyclitrophicus-A. tonsa model could be informative in future studies on Vibrio-copepod association.

Invader at the edge - Genomic origins and physiological differences of round gobies across a steep urban salinity gradient

Species invasions are a global problem of increasing concern, especially in highly connected aquatic environments. Despite this, salinity conditions can pose physiological barriers to their spread, and understanding them is important for management. In Scandinavia's largest cargo port, the invasive round goby (Neogobius melanostomus) is established across a steep salinity gradient. We used 12,937 SNPs to identify the genetic origin and diversity of three sites along the salinity gradient and round goby from western, central and northern Baltic Sea, as well as north European rivers. Fish from two sites from the extreme ends of the gradient were also acclimated to freshwater and seawater, and tested for respiratory and osmoregulatory physiology. Fish from the high-salinity environment in the outer port showed higher genetic diversity, and closer relatedness to the other regions, compared to fish from lower salinity upstream the river. Fish from the high-salinity site also had higher maximum metabolic rate, fewer blood cells and lower blood Ca²⁺. Despite these genotypic and phenotypic differences, salinity acclimation affected fish from both sites in the same way: seawater increased the blood osmolality and Na⁺ levels, and freshwater increased the levels of the stress hormone cortisol. Our results show genotypic and phenotypic differences over short spatial scales across this steep salinity gradient. These patterns of the physiologically robust round goby are likely driven by multiple introductions into the high-salinity site, and a process of sorting, likely based on behaviour or selection, along the gradient. This euryhaline fish risks spreading from this area, and seascape genomics and phenotypic characterization can inform management strategies even within an area as small as a coastal harbour inlet.

Prolonged heat waves reduce the condition index and alter the molecular parameters in the pacific oyster Crassostrea gigas

The entire shellfish farming sector is negatively affected by heat waves. Predictive models show that while heat waves are not predicted to exceed 28 °C in the northern Adriatic Sea over the coming decades, their duration will increase to periods of up to 30 days. Knowledge regarding the effects of heat waves on bivalves at physiological and molecular level is still limited. This study attempted to simulate what will happen in the future in Pacific oysters exposed to prolonged heat waves, assessing morphometric and physiological indices, and investigating the expression level of a number of genes, including the chaperone heat shock proteins HSP70, HSP72 and HSP90, and the factor P53. A state of stress in the heat wave-exposed animals was found, with loss of body weight and energy resources: despite showing a higher clearance rate, these animals were unable to absorb the nutrients required to maintain homeostasis, as well as demonstrating an alteration in hemolymphatic AST activity, total calcium and magnesium concentration. mRNA levels of all examined genes increased in response to thermal stress, with long-term overexpression, activating cell stress defense mechanisms and modulating the cycle cell. The results of this study indicate that heat waves affect oyster welfare, with consequences for the productivity of the sector due to the lack of salable products.

Proteomic changes associated with predator-induced morphological defenses in oysters

Inducible prey defenses occur when organisms undergo plastic changes in phenotype to reduce predation risk. When predation pressure varies persistently over space or time, such as when predator and prey co-occur over only part of their biogeographic ranges, prey populations can become locally adapted in their inducible defenses. In California estuaries, native Olympia oyster (Ostrea lurida) populations have evolved disparate phenotypic responses to an invasive predator, the Atlantic oyster drill (Urosalpinx cinerea). In this study, oysters from an estuary with drills, and oysters from an estuary without drills, were reared for two generations in a laboratory common garden, and subsequently exposed to cues from Atlantic drills. Comparative proteomics was then used to investigate molecular mechanisms underlying conserved and divergent aspects of their inducible defenses. Both populations developed smaller, thicker, and harder shells after drill exposure, and these changes in shell phenotype were associated with up-regulation of calcium transport proteins that could influence biomineralization. Inducible defenses evolve in part because defended phenotypes incur fitness costs when predation risk is low. Immune proteins were down-regulated by both oyster populations after exposure to drills, implying a trade-off between biomineralization and immune function. Following drill exposure, oysters from the population that co-occurs with drills grew smaller shells than oysters inhabiting the estuary not yet invaded by the predator. Variation in the response to drills between populations was associated with isoform-specific protein expression. This trend suggests that a stronger inducible defense response evolved in oysters that co-occur with drills through modification of an existing mechanism.

Transcriptomic, Proteomic, and Functional Assays Underline the Dual Role of Extrapallial Hemocytes in Immunity and Biomineralization in the Hard Clam Mercenaria mercenaria

Circulating hemocytes in the hemolymph represent the backbone of innate immunity in bivalves. Hemocytes are also found in the extrapallial fluid (EPF), the space delimited between the shell and the mantle, which is the site of shell biomineralization. This study investigated the transcriptome, proteome, and function of EPF and hemolymph in the hard clam Mercenaria mercenaria. Total and differential hemocyte counts were similar between EPF and hemolymph. Overexpressed genes in the EPF were found to have domains previously identified as being part of the "biomineralization toolkit" and involved in bivalve shell formation. Biomineralization related genes included chitin-metabolism genes, carbonic anhydrase, perlucin, and insoluble shell matrix protein genes. Overexpressed genes in the EPF encoded proteins present at higher abundances in the EPF proteome, specifically those related to shell formation such as carbonic anhydrase and insoluble shell matrix proteins. Genes coding for bicarbonate and ion transporters were also overexpressed, suggesting that EPF hemocytes are involved in regulating the availability of ions critical for biomineralization. Functional assays also showed that Ca²⁺ content of hemocytes in the EPF were significantly higher than those in hemolymph, supporting the idea that hemocytes serve as a source of Ca²⁺ during biomineralization. Overexpressed genes and proteins also contained domains such as C1q that have dual functions in biomineralization and immune response. The percent of phagocytic granulocytes was not significantly different between EPF and hemolymph. Together, these findings suggest that hemocytes in EPF play a central role in both biomineralization and immunity.

Bivalve Haemocyte Subpopulations: A Review

Bivalve molluscs stand out for their ecological success and their key role in the functioning of aquatic ecosystems, while also constituting a very valuable commercial resource. Both ecological success and production of bivalves depend on their effective immune defence function, in which haemocytes play a central role acting as both the undertaker of the cellular immunity and supplier of the humoral immunity. Bivalves have different types of haemocytes, which perform different functions. Hence, identification of cell subpopulations and their functional characterisation in immune responses is essential to fully understand the immune system in bivalves. Nowadays, there is not a unified nomenclature that applies to all bivalves. Characterisation of bivalve haemocyte subpopulations is often combined with 1) other multiple parameter assays to determine differences between cell types in immune-related physiological activities, such as phagocytosis, oxidative stress and apoptosis; and 2) immune response to different stressors such as pathogens, temperature, acidification and pollution. This review summarises the major and most recent findings in classification and functional characterisation of the main haemocyte types of bivalve molluscs.

Transcriptomic analysis of shell repair and biomineralization in the blue mussel, Mytilus edulis

BACKGROUND

Biomineralization by molluscs involves regulated deposition of calcium carbonate crystals within a protein framework to produce complex biocomposite structures. Effective biomineralization is a key trait for aquaculture, and animal resilience under future climate change. While many enzymes and structural proteins have been identified from the shell and in mantle tissue, understanding biomieralization is impeded by a lack of fundamental knowledge of the genes and pathways involved. In adult bivalves, shells are secreted by the mantle tissue during growth, maintenance and repair, with the repair process, in particular, amenable to experimental dissection at the transcriptomic level in individual animals.

RESULTS

Gene expression dynamics were explored in the adult blue mussel, Mytilus edulis, during experimentally induced shell repair, using the two valves of each animal as a matched treatment-control pair. Gene expression was assessed using high-resolution RNA-Seq against a de novo assembled database of functionally annotated transcripts. A large number of differentially expressed transcripts were identified in the repair process. Analysis focused on genes encoding proteins and domains identified in shell biology, using a new database of proteins and domains previously implicated in biomineralization in mussels and other molluscs. The genes implicated in repair included many otherwise novel transcripts that encoded proteins with domains found in other shell matrix proteins, as well as genes previously associated with primary shell formation in larvae. Genes with roles in intracellular signalling and maintenance of membrane resting potential were among the loci implicated in the repair process. While haemocytes have been proposed to be actively involved in repair, no evidence was found for this in the M. edulis data.

CONCLUSIONS

The shell repair experimental model and a newly developed shell protein domain database efficiently identified transcripts involved in M. edulis shell production. In particular, the matched pair analysis allowed factoring out of much of the inherent high level of variability between individual mussels. This snapshot of the damage repair process identified a large number of genes putatively involved in biomineralization from initial signalling, through calcium mobilization to shell construction, providing many novel transcripts for future in-depth functional analyses.

A bivalve biomineralization toolbox

Mollusc shells are a result of the deposition of crystalline and amorphous calcite catalysed by enzymes and shell matrix proteins. Developing a detailed understanding of bivalve mollusc biomineralization pathways is complicated not only by the multiplicity of shell forms and microstructures in this class, but also by the evolution of associated proteins by domain co-option and domain shuffling. In spite of this, a minimal biomineralization toolbox comprising proteins and protein domains critical for shell production across species has been identified. Using a matched pair design to reduce experimental noise from inter-individual variation, combined with damage-repair experiments and a database of biomineralization shell matrix proteins (SMP) derived from published works, proteins were identified that are likely to be involved in shell calcification. Eighteen new, shared proteins likely to be involved in the processes related to the calcification of shells were identified by analysis of genes expressed during repair in Crassostrea gigas, Mytilus edulis and Pecten maximus. Genes involved in ion transport were also identified as potentially involved in calcification either via the maintenance of cell acid-base balance or transport of critical ions to the extrapallial space, the site of shell assembly. These data expand the number of candidate biomineralization proteins in bivalve molluscs for future functional studies and define a minimal functional protein domain set required to produce solid microstructures from soluble calcium carbonate. This is important for understanding molluscan shell evolution, the likely impacts of environmental change on biomineralization processes, materials science, and biomimicry research.

Deciphering mollusc shell production: the roles of genetic mechanisms through to ecology, aquaculture and biomimetics

Most molluscs possess shells, constructed from a vast array of microstructures and architectures. The fully formed shell is composed of calcite or aragonite. These CaCO₃ crystals form complex biocomposites with proteins, which although typically less than 5% of total shell mass, play significant roles in determining shell microstructure. Despite much research effort, large knowledge gaps remain in how molluscs construct and maintain their shells, and how they produce such a great diversity of forms. Here we synthesize results on how shell shape, microstructure, composition and organic content vary among, and within, species in response to numerous biotic and abiotic factors. At the local level, temperature, food supply and predation cues significantly affect shell morphology, whilst salinity has a much stronger influence across latitudes. Moreover, we emphasize how advances in genomic technologies [e.g. restriction site-associated DNA sequencing (RAD-Seq) and epigenetics] allow detailed examinations of whether morphological changes result from phenotypic plasticity or genetic adaptation, or a combination of these. RAD-Seq has already identified single nucleotide polymorphisms associated with temperature and aquaculture practices, whilst epigenetic processes have been shown significantly to modify shell construction to local conditions in, for example, Antarctica and New Zealand. We also synthesize results on the costs of shell construction and explore how these affect energetic trade-offs in animal metabolism. The cellular costs are still debated, with CaCO₃ precipitation estimates ranging from 1-2 J/mg to 17-55 J/mg depending on experimental and environmental conditions. However, organic components are more expensive (~29 J/mg) and recent data indicate transmembrane calcium ion transporters can involve considerable costs. This review emphasizes the role that molecular analyses have played in demonstrating multiple evolutionary origins of biomineralization genes. Although these are characterized by lineage-specific proteins and unique combinations of co-opted genes, a small set of protein domains have been identified as a conserved biomineralization tool box. We further highlight the use of sequence data sets in providing candidate genes for in situ localization and protein function studies. The former has elucidated gene expression modularity in mantle tissue, improving understanding of the diversity of shell morphology synthesis. RNA interference (RNAi) and clustered regularly interspersed short palindromic repeats - CRISPR-associated protein 9 (CRISPR-Cas9) experiments have provided proof of concept for use in the functional investigation of mollusc gene sequences, showing for example that Pif (aragonite-binding) protein plays a significant role in structured nacre crystal growth and that the Lsdia1 gene sets shell chirality in Lymnaea stagnalis. Much research has focused on the impacts of ocean acidification on molluscs. Initial studies were predominantly pessimistic for future molluscan biodiversity. However, more sophisticated experiments incorporating selective breeding and multiple generations are identifying subtle effects and that variability within mollusc genomes has potential for adaption to future conditions. Furthermore, we highlight recent historical studies based on museum collections that demonstrate a greater resilience of molluscs to climate change compared with experimental data. The future of mollusc research lies not solely with ecological investigations into biodiversity, and this review synthesizes knowledge across disciplines to understand biomineralization. It spans research ranging from evolution and development, through predictions of biodiversity prospects and future-proofing of aquaculture to identifying new biomimetic opportunities and societal benefits from recycling shell products.

Dilution of Seawater Affects the Ca2 + Transport in the Outer Mantle Epithelium of Crassostrea gigas

Varying salinities of coastal waters are likely to affect the physiology and ion transport capabilities of calcifying marine organisms such as bivalves. To investigate the physiological effect of decreased environmental salinity in bivalves, adult oysters (Crassostrea gigas) were exposed for 14 days to 50% seawater (14) and the effects on mantle ion transport, electrophysiology and the expression of Ca²⁺ transporters and channels relative to animals maintained in full strength sea water (28) was evaluated. Exposure of oysters to a salinity of 14 decreased the active mantle transepithelial ion transport and specifically affected Ca²⁺ transfer. Gene expression of the Na⁺/K⁺-ATPase and the sarco(endo)plasmic reticulum Ca²⁺-ATPase was decreased whereas the expression of the T-type voltage-gated Ca channel and the Na⁺/Ca²⁺-exchanger increased compared to animals maintained in full SW. The results indicate that decreased environmental salinities will most likely affect not only osmoregulation but also bivalve biomineralization and shell formation.

Calcium-dependent site-switching regulates expression of the atypical iam pilus locus in Vibrio vulnificus

The opportunistic human pathogen Vibrio vulnificus inhabits warm coastal waters and asymptomatically colonizes seafood, most commonly oysters. We previously characterized an isolate that exhibited greater biofilm formation, aggregation and oyster colonization than its parent. This was due, in part, to the production of a Type IV Tad pilus (Iam). However, the locus lacked key processing and regulatory genes required for pilus production. Here, we identify a pilin peptidase iamP, and LysR-type regulator (LRTR) iamR, that fulfil these roles and show that environmental calcium, which oysters enrich for shell repair and growth, regulates iam expression. The architecture of the iam locus differs from the classical LRTR paradigm and requires an additional promoter to be integrated into the regulatory network. IamR specifically recognized the iamR promoter (P_iamR ) and the intergenic iamP-iamA region (P_iamP-A ). P_iamR exhibited classical negative auto-regulation but, strikingly, IamR inversely regulated the divergent iamP and iamA promoters in a calcium-dependent manner. Moreover, expression of the c-di-GMP and calcium-regulated, biofilm-promoting brp exopolysaccharide was IamA-dependent. These results support a scenario in which the calcium-enriched oyster environment triggers IamP-mediated processing of prepilin amassed in the periplasm for rapid pilin elaboration and subsequent BRP production to promote colonization.

Dominin and Segon Form Multiprotein Particles in the Plasma of Eastern Oysters (Crassostrea virginica) and Are Likely Involved in Shell Formation

Dominin and segon are two proteins purified and characterized from the plasma of eastern oysters Crassostrea virginica, making up about 70% of the total plasma proteins. Their proposed functions are in host defense based on their pathogen binding properties and in metal metabolism based on their metal binding abilities. In the present study, the two proteins were further studied for their native states in circulation and extrapallial fluid and their possible involvement in shell formation. Two-dimensional electrophoresis confirmed that the oyster plasma was dominated by a few major proteins and size exclusion chromatography indicated that these proteins were present in circulation in a morphologically homogenous form. Density gradient ultracentrifugation in Cesium Chloride isolated morphologically homogenous particles of about 25 nm in diameter from the plasma and extrapallial fluids. Polyacrylamide gel electrophoresis identified dominin, segon and an unidentified protein as the principal components of the particles and the three proteins likely formed a multiprotein complex that associated to form the particle. Additionally, three major proteins extracted from shell organic matrix were identified based on the apparent molecular weight in SDS-PAGE to correspond to the three major proteins of plasma and protein particles. Moreover, the hemocyte expression of dominin and segon genes measured by real-time RT-PCR increased significantly upon the initiation of shell repair and were significantly greater in younger oysters. These findings suggest that dominin and segon form protein particles by association with each other and perhaps some other major plasma proteins and play a significant role in oyster shell formation.

Calcium transfer across the outer mantle epithelium in the Pacific oyster, Crassostrea gigas

Calcium transport is essential for bivalves to be able to build and maintain their shells. Ionized calcium (Ca2+) is taken up from the environment and eventually transported through the outer mantle epithelium (OME) to the shell growth area. However, the mechanisms behind this process are poorly understood. The objective of the present study was to characterize the Ca2+ transfer performed by the OME of the Pacific oyster, Crassostrea gigas, as well as to develop an Ussing chamber technique for the functional assessment of transport activities in epithelia of marine bivalves. Kinetic studies revealed that the Ca2+ transfer across the OME consists of one saturable and one linear component, of which the saturable component fits best to Michaelis-Menten kinetics and is characterized by a Km of 6.2 mM and a Vmax of 3.3 nM min-1 The transcellular transfer of Ca2+ accounts for approximately 60% of the total Ca2+ transfer across the OME of C. gigas at environmental Ca2+ concentrations. The use of the pharmacological inhibitors: verapamil, ouabain and caloxin 1a1 revealed that voltage-gated Ca2+-channels, plasma-membrane Ca2+-ATPase and Na+/Ca2+-exchanger all participate in the transcellular Ca2+ transfer across the OME and a model for this Ca2+ transfer is presented and discussed.

Integrative cytological analysis of the effects of Ca2+ and vitamin D3 on extracellular Ca2+ flux and intracellular Ca2+ reserves in the mantle of the pearl oyster (Hyriopsis cumingii Lea)

To examine Ca²⁺ absorption and transportation in the freshwater pearl oyster, Hyriopsis cumingii Lea, we studied the effects of different levels of either extracellular Ca²⁺ or 1,25(OH)₂D₃ on extracellular Ca²⁺ flux and intracellular Ca²⁺ concentrations in mantle cells using the non-invasive micro-test technique and laser scanning confocal microscopy. The inner and outer mantle (IM and OM) cells from mussels were cultured and then treated with different concentrations of Ca²⁺ and 1,25(OH)₂D_3. Extracellular Ca²⁺ flux and intracellular Ca²⁺ reserves were analyzed. The results showed that both extracellular Ca²⁺ and 1,25(OH)₂D₃ had significant effects on Ca²⁺ flux and reserves in mantle cells, especially in IM cells (P < .05). The increase in extracellular Ca²⁺ concentrations resulted in the conversion of extracellular Ca²⁺ flux into influx with an increase in flow rate (P < .05). The calcium ion fluorescence intensity of OM cells was higher than that of IM cells (P < .05). 1,25(OH)₂D₃ addition also significantly increased the influx rate of extracellular Ca²⁺, especially in IM cells, which were more sensitive to 1,25(OH)₂D₃ addition and had significantly higher Ca²⁺ influx rates than did OM cells (P < .05). Fluorescence intensities of intracellular Ca²⁺ first increased and then decreased with increasing 1,25(OH)₂D₃ levels. The study showed that IM cells play an important role in absorbing Ca²⁺ from the environment, while OM cells mainly function in the temporary storage and transportation of Ca²⁺ in the body. The current results suggested that high levels of extracellular Ca²⁺ (1.25 mM) or 1,25(OH)₂D₃ (over 100 IU/L) were favorable for Ca²⁺ uptake and maintenance in the body.

Environmental Calcium Initiates a Feed-Forward Signaling Circuit That Regulates Biofilm Formation and Rugosity in Vibrio vulnificus

The second messenger c-di-GMP is a key regulator of bacterial physiology. The V. vulnificus genome encodes nearly 100 proteins predicted to make, break, and bind c-di-GMP. However, relatively little is known regarding the environmental signals that regulate c-di-GMP levels and biofilm formation in V. vulnificus. Here, we identify calcium as a primary environmental signal that specifically increases intracellular c-di-GMP concentrations, which in turn triggers brp-mediated biofilm formation. We show that PAPS, a metabolic intermediate of the sulfate assimilation pathway, acts as a second messenger linking environmental calcium and sulfur source availability to the production of another intracellular second messenger (c-di-GMP) to regulate biofilm and rugose colony formation, developmental pathways that are associated with environmental persistence and efficient bivalve colonization by this potent human pathogen.

Poor clinical outcomes (disfigurement, amputation, and death) and significant economic losses in the aquaculture industry can be attributed to the potent opportunistic human pathogen Vibrio vulnificus. V. vulnificus, as well as the bivalves (oysters) it naturally colonizes, is indigenous to estuaries and human-inhabited coastal regions and must endure constantly changing environmental conditions as freshwater and seawater enter, mix, and exit the water column. Elevated cellular c-di-GMP levels trigger biofilm formation, but relatively little is known regarding the environmental signals that initiate this response. Here, we show that calcium is a primary environmental signal that specifically increases intracellular c-di-GMP concentrations, which in turn triggers expression of the brp extracellular polysaccharide that enhances biofilm formation. A transposon screen for the loss of calcium-induced P_brpA expression revealed CysD, an enzyme in the sulfate assimilation pathway. Targeted disruption of the pathway indicated that the production of a specific metabolic intermediate, 3′-phosphoadenosine 5′-phosphosulfate (PAPS), was required for calcium-induced P_brpA expression and that PAPS was separately required for development of the physiologically distinct rugose phenotype. Thus, PAPS behaves as a second messenger in V. vulnificus. Moreover, c-di-GMP and BrpT (the activator of brp expression) acted in concert to bias expression of the sulfate assimilation pathway toward PAPS and c-di-GMP accumulation, establishing a feed-forward regulatory loop to boost brp expression. Thus, this signaling network links extracellular calcium and sulfur availability to the intracellular second messengers PAPS and c-di-GMP in the regulation of V. vulnificus biofilm formation and rugosity, survival phenotypes underpinning its evolution as a resilient environmental organism.